1. Field of the Invention
The present invention relates to spherical shaped crystals of salt obtained from saturated brine useful for edible and non-edible applications.
More particularly, the present invention relates to spherical shaped crystals of salt having size in the range of 100-500 μm obtained from saturated brine without using any habit modifier.
The present invention further relates to a process of preparation of spherical shaped crystals of salt having size in the range of 100-500 μm obtained from saturated brine without using any habit modifier.
2. Background Information
Common salt or sodium chloride or NaCl, apart from being an essential dietary component, is a basic raw material for the manufacture of a wide variety of industrial chemicals viz. sodium carbonate (soda ash), sodium hydroxide (caustic soda), and chlorine. Besides, salt is used in textile, dairy, dyeing, food, fertilizer, leather, paper and pharmaceutical industries. The flow properties of salt are important in many applications that entail use of granular salt.
Reference may be made to the article “What is a Granular Medium?” by Granular Volcano Group (world-wide web at address granular-volcano-group.org; and granular.org), wherein it is stated that the concentrated flow of granular substances can be studied by measuring the angle of repose (or angle of internal friction). It is stated therein that the angle of repose is typically in the range of 15 o to 50 o and that the angle is low when grains are smooth, coarse or rounded, and, it is high for sticky, sharp, irregular, or very fine particles.
Reference may also be made to the paper “The effect of filler size and geometry on the flow of carbon black filled rubber” by P. P. A. Smit (Rheologica Acta, Volume 8, Number 3/August, 1969).
Reference may be made to any standard text book in the area of solid state chemistry or physics wherein it is mentioned that common salt crystallizes with cubic morphology.
Reference may be made to the well known prior art wherein in many solar salt pans nearly round-shaped salt can be seen at the edges of the crystallizers. However, these salt granules are very large, measuring 2-4 mm which would make it unsuitable for many applications.
Reference may be made to the paper by R. J. Davey et al. (J. Cryst. Growth, 1991, vol. 114, pp. 7-12) in which the structures of different types of salt crystals are described. A figure is also shown of granular NaCl with 1 mm size. Neither is the process of preparation disclosed nor is any mention made of spherical salt of the size range of the present invention.
Reference may be made to the Japanese Patent No: JP63162526A2 wherein a process of producing spherical salt is depicted. In the said process conventional salt crystals having dimension of 0.5-0.8 mm are subjected to centrifugal force. A binding agent is then added followed by spraying of fine salt particles passing through 250 mesh (i.e., about 70-80 micron) to yield spherical salt. The main problem with the process is that the resultant particles of spherical granular salt would be bigger than the size of the nucleus taken which itself is rather big. Another problem is that the round salt cannot be directly obtained from brine but only from further processing of pre-formed salt. Moreover, the process requires use of a very fine salt powder to coat over the initial salt nucleus and making such fine salt has its own associated problems.
Reference may be made to the European Patent No: EP 1,545,733,B9 (WO 2004/018068) dated Apr. 21 1999 by Mayer et al. wherein an evaporative salt crystallization process that produces pure salt of octahedral or spherical morphology is disclosed. The process utilized saccharide or its derivative in an evaporative process occurring at room temperature. The main disadvantage of the said process is that the saccharide is used in about 5% (w/v) concentration. Such high concentrations of saccharides would (i) lead to unwarranted increase in viscosity of the brine which, in turn, would slow down evaporation, (ii) result in contamination of the salt with saccharide, and (iii) add to cost. Moreover, the process may not be suitable for salt production by forced evaporation at elevated temperatures since the example provided is only for ambient temperature evaporation. No mention is made of the specific sizes of spherical crystals obtained nor is there any quantitative measurement reported of improved flow.
Reference may be made to U.S. Pat. No. 3,567,371 dated Mar. 2, 1971 by Birchall et al. wherein the preparation of a novel form of NaCl crystal is depicted. The crystallization is carried out from saturated sodium chloride solution containing certain additives. The novel form of crystal is formed in the presence of polyvinyl alcohol alone or in association with polyelectrolytes. Although the process produced an interesting form of NaCl, it does not deal with spherical-shaped salt crystals.
Reference may be made to Japanese Patent No: JP 19870003 13145 dated Dec. 12, 1987 wherein sodium chloride having flat tetrahedron-shaped crystal form is produced by treating an aqueous solution of sodium chloride with a specified catalyst. The invention nicely explains the effect of additive on the growth of various planes and hence, the change in habit or morphology of the crystals. No mention is made of any attempt to produce spherical morphology of salt crystals.
Reference may be made to the preparation of spherical sodium chloride having 6-50 nm (world-wide web at address seas.harvard.edu/environmental-chemistry/projects/aerosolsub3.php) size. Although such salt is useful for fundamental studies, it is not of much use in conventional dietary and industrial applications. Besides, its production in bulk quantity is difficult and costly.
Reference may be made to the Japanese Patent No: JP09086923A2 where in particles of sodium chloride and/or potassium chloride are brought into contact in suspended state with the flame of a burner or a hot (≧800° C. and preferably 1000-1300° C.) gas, to effect the partial melting of the salt particle followed by its cooling. The particle of the salt solidified by cooling is a glassy single particle having nearly true spherical form and particle diameter of 0.01-1.0 mm. The main disadvantage of the process is that the process cannot be considered as practical or cost-effective for bulk production of spherical salt. Neither is there any mention of the control exercised over the size distribution.
Reference may be made to U.S. Pat. No. 7,220,435 by Dastidar et al. wherein the importance of crystal morphology and means of achieving the same for NaCl are described. The patent further discloses the preparation of NaCl of dodecahedron shape. The process entails use of large concentrations of glycine which acts as a habit modifier. Moreover, the process is not applicable at elevated temperatures and, most importantly, the salt crystals are not spherical in a true sense since a rhombic dodecahedran has 12 flat surfaces which would make it difficult for the salt to roll freely. There is also no mention of the size of crystals obtained.
Reference may also be made to the paper by Ballabh et al. (Cryst. Growth & Des., 2006, Vol. 6 (No. 7), p 1591) which provides scientific insight into habit modification of common salt with glycine and further reports changes in angle of repose achieved by converting cubic salt into rhombic dodecahedron shape. There is only a small improvement in flow characteristics.
Reference may be made to the Japanese Patent No: JP2001213970-A wherein a crystallizer with special type of stirrer is used to produce spherical fine particles. The production of a dispersion liquid containing spherical fine particles has been effected by imparting a shear force to a mixture of fused or softened thermoplastic resin and polysiloxane which is not reactive with the fused or softened thermoplastic resin and does not dissolve the fused or softened thermoplastic resin. Spherical fine particles made from the dispersion are used as a powder moulding material, a sintering forming material, a filler for thermoplastic or thermoset resin, a filler for paint, a filter medium, an absorbent for chromatography columns, a spacer toner for liquid crystals, powdered paint and cosmetics. The method readily provides spherical fine particles of thermoplastic resin with high quality. An independent claim is also included for an apparatus for continuously producing dispersion liquid containing spherical fine particles comprising a container with a stirrer with a high speed shear force, a port for charging thermoplastic resin, an outlet for thermoplastic resin, and a supply port for polysiloxane. The port for charging thermoplastic resin is positioned in the vicinity of the stirrer with high speed shear force. No mention is made of preparation of any inorganic solids through this process.
Reference may be made to U.S. Pat. No. 5,366,514 and other references which deal with preparation of common salt through forced evaporation with or without application of vacuum.
Reference may be made to the U.S. Pat. No. 3,647,396 wherein the inventors have disclosed the recrystallization of sodium chloride in the form of high purity cubic crystals from a sodium chloride source containing calcium sulphate impurity by multi-effect evaporation preceded by treatment of the hot sodium chloride saturated brine by flocculants and settling chloride eliminating the conventional requirement for filtering the hot brine.
Reference may be made to vacuum evaporated commercial table salt available commercially in the Indian market (FIG. 1). The salt crystals are observed to be of cubic shape under the microscope and the size of crystals is in the size range of 200-500 μm.
Reference may be made to any standard text book wherein force-evaporated salt is produced with or without application of vacuum at elevated temperatures and normally under agitation.
Reference may be made to the Japanese Patent No: JP01212213A2 by Takehiko wherein controlling of salt crystallisation is depicted. The process used a cycle where the crystallization is carried out by heating can in which mother liquid is extracted from bottom and returning liquid to can while supplying seed crystal and brine. To stably control the growth rate of a crystal and to improve the operation efficiency by supplying brine in the amount larger than the amount of brine to be heated and evaporated at the lower-limit level of a crystallizer, and stopping the supply of brine at the upper-limit level.
Reference may be made to Hasegawa and Masaoka on the world-wide web at address saltscience.or.jp/kenkyu/jyoseilist/ENGsum/04A6-E.pdf. The authors have discussed the effect of mother liquor composition on sodium chloride crystal quality and the effect of crystal growth rate on the quality of crystals. The authors have further indicated a correlation between crystal size and crystal shape under agitated conditions, wherein salt crystals having size less than 300 micron are cubic, those having size more than 500 micron are spherical while in the range of 300-500 micron “condensation and wear” were observed. The main drawback with the prior art is that no mention is made of process parameters and, more importantly, it precludes the possibility of producing salt with good sphericity when the size is <500 micron, whereas many applications of interest demand salt in this size range.
Reference may be made to the European Patent No: EP 0,909,574, A1 dated Apr. 21, 1999 by R, Moschini et al. where in a method for producing salt grains having size distribution preferably in the range of 100-300 μm has been disclosed. The said process is based on atomization of a supersaturated solution of salt in a chamber where a stream of hot air circulates. The process further claimed that the size of nearly spherical shape salt crystals depends on the atomization process and could be controlled in the desired sire range. Although the process can produce spherical salt with size distribution in the range of 100-300 μm and the product is claimed as having homogeneous characteristics, the method suffers from a great disadvantage of operational complications. The process is also not cost effective.
It would be evident from the above prior art that no practical process is reported thus far for the preparation of spherical sodium chloride in the size range of 200-500 μm even though such size range of salt is evidently important in day-to-day products such as vacuum evaporated salt.
Reference may be made to the research article by Zijlema et al. (separation and purification by crystallization ACS symposium series 667: 230-241 1997) wherein the suitability of the amines, diisopropylamine (DiPA) and dimethylisopropylamine, (DMiPA) as anti-solvents for the crystallization of sodium chloride from its aqueous solution has been demonstrated. Continuous crystallization experiments were carried out at temperatures below the liquid-liquid equilibrium line in the single liquid phase area. The product consisted of cubic agglomerated NaCl crystals with maximum primary particle sizes of 10-70 μm.
Reference may be made to U.S. Pat. No. 3,770,390 dated Nov. 6, 1973 by Toet et al. wherein an improved method for the crystallization of water soluble inorganic salts is disclosed. The method utilised monovalent salt of polymeric sulphonate or sulphate additive. The resulting crystals are larger and more regularly shaped. The main problem of the method is that it does not deal with spherical shape of the resulting product. Moreover, the crystals are more than millimetre in size.
Reference may be made to the US Patent No: 2005/0206022A1 by Pelikann et al. wherein the authors depicted a process for the preparation of small particles through precipitation. The patent further disclosed the use of non gaseous anti solvent for the preparation of small particles from a saturated solution of the solute which is to be precipitated. Particles having size distribution in the range of 0.1-80 μm, and generally with cubic morphology, are reported.
Reference may be made to the U.S. Pat. No. 6,621,355 B2 dated Sep. 16, 2003 by Gupta et al. where in a novel way to produce very small particles in the nanometer range, having a narrow size distribution have been disclosed. The process of generation of nano-particles can be extended to a wide variety of materials. The said process used a supercritical fluid as an anti-solvent. Additionally, the dispersion jet generated from the solution containing supercritical anti-solvent is deflected by a vibrating surface that atomises the jet into micro droplets. The main advantage of the prior art is the ability to produce particles in nano-meter dimension with very narrow side distribution as a result of uniform droplet atomization. Also the prior art has an edge over other similar processes in that it can control the size of the particle by changing the vibration intensity of the deflecting surface. However, the main disadvantage of the prior art is the requirement of very high frequency vibration for atomization. Also the particle size distribution varies in the range of nanometer to few micrometer, which may have special application but not of the kind envisaged in the present invention.
Reference may be made to the U.S. Pat. No. 4,263,011 dated Apr. 21, 1981 by Huguenard et al. where in a process of producing fine crystals having improved homogeneity and narrow particle size distribution is disclosed. The crystallization in the said process has been carried out by introducing a solution of a crystallisable solute in a solvent into a bed of small inert continuously moving solid particles and initiating crystallization in the solution while it is in contact with the solid moving bed. The process of the prior art is useful for preparation of morphologically homogeneous inorganic materials with narrow size distribution. The process does not deal with the preparation of spherical shape crystals and also involves several complicated steps.
Reference may be made to the Oslo crystallizer which is a well-known apparatus for the crystallization of inorganic substances, including NaCl, as described in British Chemical Engineering, Vol. 16, pp 681-685, 1971 and British Patent GB-A-418,349. This known apparatus comprises a vertical cylindrical vessel and a vertical tube which is arranged axially in the vessel and which opens in the immediate vicinity of the bottom of the latter; a vertical annular chamber is thus defined between the axial tube and the cylindrical wall of the vessel. In making use of this known apparatus, a bed of crystals is employed in the annular chamber, through which passes a solution supersaturated with the substance which it is desired to crystallize, which substance includes NaCl. This solution is introduced into the apparatus via the axial tube, so that it enters the annular chamber radially, near the bottom of the latter, and subjects the crystals in the bed to a general rotation comprising an upward translation along the wall of the vessel and a downward translation along the axial tube. A great advantage of the above said process in crystallizers, where intensive production of large quantity of granular solids from supersaturated liquid is sought for, is to allow the growth of granular solid individually from supersaturated liquid without causing cake formation. There is no mention of preparation of spherical salt having the size range of 200-500 μm.
Reference may be made to the U.S. Pat. No. 6,478,828 B1 dated Nov. 12, 2002 by Léon Ningana et al. wherein a process is depicted for the crystallization of inorganic salt from supersaturated solution. The process used a bed of crystals which is fluidized by passing the supersaturated solution of the inorganic solute through a distributor which is arranged below the bed of crystals and maintained at a suitable temperature. The bed of crystals in the prior art acts as seeds for the crystallization of the inorganic material. The said process in the invention allows the inorganic substance to crystallize in the form of uniform particles of nearly spherical shape which are generally monolithic spherical beads and having size in the range of 3-30 mm. The main drawback in the context of the present invention is that crystals of smaller size are not obtained through this process.
Reference may be made to the paper by R. Reverchon (Ind. Engg. Chem. Res. Vol. 41, pp 2405-2411, 2002) wherein supercritical CO2-assisted atomization technique has been disclosed to produce micro and nano particles of solid with controlled size. The process is based on the solubilization of controlled quantity of supercritical CO2 in liquid solutions containing a solid solute and subsequent atomization of the ternary solution through a nozzle. The process is reported to be versatile and can be used for any kind of solid using various types of solvents. The said process can produce solid particles in the size range of 0.1-3 μm.
Reference may be made to the paper by S. Kaneko et al. (J. Chem. Engg. Japan Vol. 35, pp. 1219-1223, 2002) wherein effect of ethanol as an anti-solvent on the crystallization of NaCl have been reported. It is shown that addition of ethanol enhances the local supersaturation greatly. The authors have proposed a new method to diminish the high local supersaturation by using very high concentration of ethanol. The said process is reported to yield unagglomerated and monodispersed crystals at an optimum antisolvent concentration. Is has been further shown that nucleation induced by anti-solvent addition occurred by the change in the local supersaturation at the boundary of the starting and feed solution. A correlation between the number of crystals and local supersaturation created by the anti solvent is described. Only cubic salt particles are reported.
Reference may be made to the paper by A. Mersmann et al. (Chem. Engg. Tech., CET Vol 12, pp 137-146, 2004) wherein secondary nucleation in industrial crystallizers is shown to depend on both supersaturation and mechanical stress by stirring. Most models which consider mechanical stress assume that nucleation is proportional to the energy transferred to the crystals during collisions. This is not based on any physical relationship and, in addition, the models do not satisfactorily reproduce the experimental results. Own model, based on the theory of Hertz/Huber, which accounts for the stress of the crystals caused by impact, gave better results. This well-known and proven theory allows the calculation of the volume abraded during collisions between crystals and stirrer or walls. Introducing a nucleate efficiency, the effect of mechanical stress on the rate of secondary nucleation, due to stirring intensity and crystallizer size, can be determined.
Reference may be made to the European Patent No. WO 2006/045795 dated May 4, 2006 by Bargeman et al. wherein a process for the crystallization of salt using antisolvent is disclosed. The proposed process is claimed to work in a close loop to produce salt crystals as well as pure drinkable water through nano-filtration. The effect of the presence of crystal growth inhibitor and anti solvent is also disclosed.
Reference may be made to the U.S. Pat. No. 625,031 dated May 16, 1899 by L. Hirt wherein a new crystallizer and more particularly an apparatus for crystallizing sugar from its mother liquor is disclosed. The apparatus is claimed to have the capability of crystallizing out all the substances those are present in the mother liquor from where sugar is crystallized out. Further, the apparatus is specifically good for the crystallization of saccharine. However, the apparatus does not say anything about the morphology modification of the resulting crystals.
Reference may be made to the U.S. Pat. No. 1,932,364 B1 dated Oct. 24, 1933 by Otto V. Martin wherein an apparatus for the preparation of anhydrous metallic chlorides such as CaCl2 or MgCl2 and the like is disclosed. There is no mention of the morphology of particles obtained.
Reference may be made to the U.S. Pat. No. 2,458,450 dated Jan. 4, 1949 by J. W. Stafford wherein a new crystallizer and more specifically an apparatus for completing the growth of sugar crystals in its mother liquor and a provision for reheating the mother liquor is disclosed. The crystallizer helps to uniform growth of the sugar crystals by ensuring uniform heat exchange with continual motion of the mother liquor in a direction to minimise its exhaustion effect on the crystal growth. The process, however, does not reveal any information on the morphology of the resulting sugar crystals.
Reference may be made to the U.S. Pat. No. 1,593,564 dated Jul. 20, 1975 by R. Lafay et al. wherein a process for selectively crystallizing one of the constituents of a liquid mixture of at least two components is disclosed. The process carried out by cooling down the mixture by direct thermal exchange with an immiscible liquid coolant. The formation of the crystals of the crystallisable component takes place in the stirring zone whereas the partial separation of the crystals takes place in the quiet zone. The main disadvantage of the process is that it does not say anything about the morphology of the resulting crystals.
It will be evident from the prior art that although anti-solvent effects have been utilized to produce particles of very small size, there is no mention of use of this approach for producing spherical salt from brine with the larger proportion of crystallized salt having size range of 200-500 micron.